Get the intersection point, if any, between two infinite lines.

getLineLineIntersection.ts

export function intersectLineLine(AB: number[][], PQ: number[][]): number[] | undefined {
  const slopeAB = AB[0][0] === AB[1][0] ? NaN : (AB[0][1] - AB[1][1]) / (AB[0][0] - AB[1][0])
  const slopePQ = AB[0][0] === PQ[1][0] ? NaN : (PQ[0][1] - PQ[1][1]) / (PQ[0][0] - PQ[1][0])

  if (slopeAB === slopePQ) return undefined

  if (Number.isNaN(slopeAB) && !Number.isNaN(slopePQ)) {
    return [AB[0][0], (AB[0][0] - PQ[0][0]) * slopePQ + PQ[0][1]]
  }

  if (Number.isNaN(slopePQ) && !Number.isNaN(slopeAB)) {
    return [PQ[0][0], (PQ[0][0] - AB[0][0]) * slopeAB + AB[0][1]]
  }

  const x = (slopeAB * AB[0][0] - slopePQ * PQ[0][0] + PQ[0][1] - AB[0][1]) / (slopeAB - slopePQ)
  const y = slopePQ * (x - PQ[0][0]) + PQ[0][1]

  return [x, y]
}